The molecular basis for sarcomere organization in vertebrate skeletal muscle

Zhexin Wang, Michael Grange, Thorsten Wagner, Ay Lin Kho, Mathias Gautel, Stefan Raunser - Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology ; The Randall Centre for Cell and Molecular Biophysics, School of Basic and Medical Biosciences, Kings College London

Sarcomeres are force-generating and load-bearing devices of muscles. A precise molecular picture of how sarcomeres are built underpins understanding their role in health and disease. Here, we determine the molecular architecture of native vertebrate skeletal sarcomeres by electron cryo-tomography.

Our reconstruction reveals molecular details of the three-dimensional organization and interaction of actin and myosin in the A-band, I-band, and Z-disc and demonstrates that α-actinin cross-links antiparallel actin filaments by forming doublets with 6-nm spacing. Structures of myosin, tropomyosin, and actin at ~10 Å further reveal two conformations of the “double-head” myosin, where the flexible orientation of the lever arm and light chains enable myosin not only to interact with the same actin filament, but also to split between two actin filaments. Our results provide unexpected insights into the fundamental organization of vertebrate skeletal muscle and serve as a strong foundation for future investigations of muscle diseases.

How Amira-Avizo Software is used

In order to visualize the organization of cross-bridges between thin and thick filaments, the densities of a complete A-band were annotated manually using Amira (Thermo Fischer Scientific) on a slice-by-slice basis. A user-defined mask based on gray values was used during manual annotation to prevent the picking of weaker densities. The 2D annotation stacked to form a 3D segmented volume depicting the arrangement of thin and thick filaments as well as the cross-bridges formed by myosin heads.